1. Field of the Invention
The present invention relates to a cylindrical radial gap type motor structure and more particularly to a cylindrical radial gap type motor structure reduced in size, weight and cost.
2. Description of the Prior Art
Recently, the performance and especially speed of office automation devices has been increasing rapidly, and in order to support the required functions of these devices it has become common to employ a plurality of dedicated motors. Particularly, in the case of small DC motors, enhanced magnetic efficiency to better support required applications, diminished cogging torque and electromagnetic noise, and superior controllability are desired. Measures adopted so far to meet these demands include increasing the number of slots (salient poles) of the core, and increasing the number of poles of the magnet in order to improve the electromagnetic efficiency by shortening each magnetic circuit to decrease magnetic resistance. Further, the winding efficiency of the coil is increased by using a concentrated winding
FIG. 7 shows the construction of a laminated, inner rotor type brushless DC motor, and FIG. 8 is the sectional view thereof. In these figures, the numeral 1 denotes a lead wire and numeral 2 denotes a coil. The coil 2 is constituted of magnet wire wound around a bobbin 9 vertically fitted on each salient pole 10 of a stator 8 which is formed by lamination of soft magnetic steel plates. Numeral 3 denotes a rotor magnet, and numeral 4 denotes a sleeve which connects the rotor magnet and a shaft 5 which acts as the rotational center of the rotor. Numeral 7 denotes a flange with a bearing 6 disposed centrally, which supports the rotor shaft 5. Two flanges 7 hold the stator 8 from both sides. The tip of each salient pole 10 is formed wider than the winding portion to increase interlinkage flux with the rotor magnet and hold the bobbin (coil). For the same reason, the laminated length of the stator 8 is made almost equal to the length of the rotor magnet 3.
However, as the number of slots increases, the slot spacing becomes narrower and hence the space available for the winding is not great enough for the required volume of winding unless the overall size of the motor is increased. If the size of the motor remains the same, the required volume of winding is not possible, and so a permanent magnet with strong magnetic characteristics is employed to obtain the desired motor characteristics, even though this leads to higher cost. In the construction of the conventional lamination type stator, it is required that the lamination thickness of the stator be made approximately equal to the length of the rotor magnet in order to ensure a predetermined area of opposition between the salient poles and the rotor magnet. As a result, the winding length for each turn of coil becomes larger and thus the winding efficiency is low. Besides, since the surface of each salient pole opposed to the rotor magnet is wide to ensure the aforesaid area of opposition and to hold the associated coil, the winding work efficiency is poor. This tendency is conspicuous particularly in the case of an inner rotor type motor. Further, the motor is heavy and difficult to handle because the stator is larger than necessary.
The present invention has been accomplished in view of the above-mentioned problems and it is an object of the invention to provide a cylindrical radial gap type motor which permits reduction in weight and greater efficiency in the same size as the prior art without using a laminated structure where the cylindrical stator is laminated in an axial direction.